Numerical Analysis of Oxy-Coal Combustion System Burning Pulverized Coal Mixed with Different Flue Gas Mass Flow Rates

A major problem for pulverized coal combustion is emissions such as ${\mathrm{CO}}_2$, CO, and ${\mathrm{NO}}_x$. In the oxy-fuel combustion method, a ${\mathrm{CO}}_2─{\mathrm{H}}_2\mathrm{O}─{\mathrm{O}}_2─{\mathrm{N}}_2$ mixture is used for the pulverized coal combustion. In oxy-fuel combustion systems, fuel is combusted in pure oxygen rather than air. Thus, the oxy-fuel combustion process produces low CO and ${\mathrm{NO}}_x$ emissions while air is not suitable for combustion in the oxy-fuel process. In this study, the oxy-fuel combustion method has been investigated for different fuel (pulverized coal) mass flow rates and different mixture recycle rates in terms of ${\mathrm{O}}_2\%/{\mathrm{CO}}_2\%/{\mathrm{H}}_2\mathrm{O}\%$, ${\mathrm{N}}_2\%$ in a lab-scale furnace. The oxy-fuel combustion method has also been investigated for different mixture rates burned of pulverized coal in the lab-scale furnace. In this analysis, the effect of different recirculated flue gas (FG) mixture rates, in terms of ${\mathrm{CO}}_2$, ${\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{O}}_2$ mass/molar fractions, on the flame temperature and ${\mathrm{CO}}_2/{\mathrm{NO}}_x$ emissions was investigated. Numerical analyses using computational fluid dynamics (CFD) of the pulverized coal combustion for different fuel mass flow rates and the FG mass flow rates have been conducted using commercial software. CFD analysis of the oxy-fuel combustion process has been performed by using a realizable $k_{\epsilon }$ turbulence model, multiple surface reactions of char combustion, single rate devolatilization, discrete ordinates (DO) radiation and second-order upwind discretization methods. The firing system of the furnace consists of a three-stage combustion burner. Through the primary stage, coal and recirculated FG streams are injected, while the secondary and tertiary stages inject oxidant medium containing 95% mass fraction and 5% mass fraction ${\mathrm{N}}_2$. The secondary stage fluid flow is a jet flow while the tertiary stage flow is a swirl flow. The effect of different recirculated FG mixture rates was investigated, in terms of ${\mathrm{CO}}_2\%$, ${\mathrm{H}}_2\mathrm{O}\%$, ${\mathrm{O}}_2\%$ ($0/0/21$, $59.5/37.5/3$, $77.6/19.4/3$, $97/0/3$) mass/molar fractions, on the flame temperature and ${\mathrm{CO}}_2/{\mathrm{NO}}_x$ emissions.